Paper laminates are products widely used in the construction industry for making counter tops and the like. They are ordinarily composed of paper impregnated with TiO.sub.2 pigment and imbedded in a polymer matrix. Such laminates have an unfortunate tendency to discolor on exposure to ultraviolet light, and this detracts from their value because in many uses they are necessarily exposed to such radiation.
The pigment of the present invention is highly resistant to discoloration on exposure to ultraviolet light, and, when used in making a paper laminate, gives a product with significantly better lightfastness than one made with conventional TiO.sub.2 pigment.
The pigment of the invention is composed of rutile TiO.sub.2 particles conventionally coated with alumina or with alumina-silica. The coating ordinarily constitutes 1-8%, by weight, of the pigment. Associated with the pigment particle surfaces are cerium cations (preferably Ce.sup.+4 ions), at a concentration of .[.0.5-2%,.]. .Iadd.about 0.17-2%, .Iaddend.by weight, and a stoichiometric amount of sulfate-, phosphate or silicate anions. "Associated with the particle surfaces" means that the ions are bound to the TiO.sub.2 particles by a chemical or physical attraction.
The ion content of the pigment may be determined by ion plasma chromatography.
The amount of alumina the TiO.sub.2 particles bear as coatings, expressed as percent by weight, is calculated by first determining, by ion plasma spectroscopy,
(a) the alumina content of a pigment of the invention, and PA1 (b) the alumina content of the raw TiO.sub.2 from which the pigment is made,
and then computing the alumina coating content of the pigment by difference.
The cerium cations may be supplied by any water-soluble cerium salt, preferably cerium sulfate, and even more preferably ceric sulfate: when the anion is sulfate, this will also supply the sulfate ion. When the anion of choice is phosphate or silicate, it is supplied by using phosphoric acid or sodium silicate solution during preparation of the pigment, as will be described.
The TiO.sub.2 used to prepare the pigment or the invention can be of the conventional rutile variety, prepared by either the chloride or the sulfate process.
The pigment of the invention can be made by first preparing an aqueous slurry containing 200-400 grams per liter of TiO.sub.2. This slurry is brought to 45.degree.-70.degree. C. and is held there during the rest of the preparation procedure. To the slurry is then added, with stirring, enough of a cerium salt to give a cerium ion concentration in the slurry of .[.0.5-2%,.]. .Iadd.0.05-2%, .Iaddend.by weight of the TiO.sub.2. If one desires a phosphate anion or a silicate anion to be associated with the pigment, he also adds a stoichiometric amount of phosphoric acid or sodium silicate. In any case, the slurry is then stirred for 1-15 minutes.
Alumina is then precipitated on the TiO.sub.2 particles by slowly adding enough sodium aluminate to the slurry to give a concentration of about 2-8%, by weight of the TiO.sub.2. Acid, ordinarily sulfuric or hydrochloric, is added at the same time to maintain the pH of the slurry within the range 6-9 during the precipitation step. After addition of the aluminate is complete, the slurry is allowed to cure, with stirring, for 15-30 minutes.
The resulting pigment is then separated from the liquid by filtration or centrifugation, is washed with water and then dried.
The pigment thus prepared may be used to prepare a paper laminate in any of the customary ways typically by mixing the pigment with paper pulp and then making paper of it in the usual way, impregnating this paper with resin, ordinarily a melamine-formaldehyde or urea-formaldehyde resin, and then curing the mixture with heat. These procedures are set forth in greater detail in U.S. Pat. No. 4,239,548, which is incorporated into this specification to show such procedures.
Coating compositions may be prepared with the pigments of the invention in any of the conventional ways, by blending the pigment with a film-forming component and a liquid carrier.